The wiki mentioned having the option to keep Reaha as a slave if she has 60% confidence or less when she pays her debt, but after curing her there are only 2 opportunities to drop her confidence and they're both only worth 5%. So even if Reaha is cured at the minimum 75% confidence its physically impossible to unlock the slave option. There needs to be a repeat opportunity to drop her confidence like there was when she was still addicted.